This invention relates to the continuous cladding of aluminum strip, and more particularly to methods of continuously cladding one or both major surfaces of an aluminum core strip with an aluminum cladding strip or strips. The term "aluminum," as used herein, embraces alloys of aluminum as well as pure aluminum metal.
For various purposes, it is frequently desired to provide aluminum sheet or strip with an outer cladding layer of aluminum on one or both major surfaces thereof. This may be done, for example, to achieve a product affording the combined advantages of particular properties of the substrate or core metal and particular surface characteristics of the cladding. Commonly, the core and cladding are respectively constituted of different alloys, although in some cases it would be desirable to provide a core and cladding of the same alloy, e.g. to enable utilization of the core in applications for which the surface condition of the core is unacceptable.
Cladding has conventionally been performed by placing a rectangular plate of the cladding alloy on one or both major surfaces of a rectangular ingot of the core alloy and warm rolling the plate or plates and ingot together to produce a clad slab which can thereafter be reduced to a desired final gauge by further rolling operations. Satisfactory bonding of the core and cladding can be achieved in this way, but the procedure is slow and time-consuming. The advent of continuous strip casting as a production technique has made especially desirable the performance of cladding in a continuous manner, e.g. in line with the strip caster, to diversify the potential end uses of continuously strip-cast products and to take advantage of the benefits (from a production standpoint) of the continuous operation afforded by strip casting.
Continuous strip casting, as herein contemplated, involves supplying molten metal to a casting zone defined between more or less closely spaced, externally chilled, moving endless surfaces such as the surfaces of continuously driven steel belts or rolls. The metal emerges from the casting zone as a continuous, relatively thin strip, at least externally solidified. The as-cast strip is reduced by passage through a hot-rolling mill in line with the caster, or if sufficiently thin it may be directly cold rolled without preliminary hot reduction. In any event, the small thickness of the as-cast strip advantageously decreases the extent of hot reduction necessary for fabrication of sheet products, as compared with conventionally cast sheet ingot. Examples of currently commercially available strip casting apparatus are Hazelett twin-belt casters and Hunter-Douglas twin-roll casters.
It has been proposed, for example in U.S. Pat. No. 4,213,558, to clad continuously cast aluminum strip by bringing thin-gauge rolled aluminum cladding strip into contact with one or both major surfaces of as-cast core strip and passing the core and cladding strip together through a warm-rolling mill. In the procedure specifically described in the aforesaid patent, the core and cladding strips are brought into contact between pinch rolls, and are fed synchronously to the mill after being maintained in extended contact ahead of the mill (for example by passage between arrays of rolls) to achieve thorough preheating of the cladding strip or strips by the hot as-cast strip.
Heretofore, however, the continuous cladding of cast strip has been attended with serious problems, related to the requirements that the cladding strip or strips be maintained in proper alignment with the core strip, that the cladding strip or strips be in smooth continuous contact with the core strip when the strips pass through the warm rolling mill, and that the cladding strip or strips be uniformly bonded to the core by the warm-rolling operation. Curvature or other deformation of the cladding strip as paid out from a coil, and local or general bonding failures (whether resulting from such deformation, or from the layer of oxide unavoidably present on aluminum surfaces, or from other causes) lead to production of excessive scrap. Inhibition of bonding by the oxide layer is particularly acute in the case of certain types of alloys (such as magnesium-containing alloys) and certain core-cladding alloy combinations, and has thus limited the range of alloys and core-cladding alloy combinations to which known continuous cladding procedures are applicable.
It has also been proposed, for example in U.S. Pat. Nos. 3,381,366 and 3,496,621, to clad an aluminum core strip by warm rolling with unheated cladding strips that first come into contact with the preheated core in the nip of the warm-rolling mill (being in contact with the mill rolls before they are in contact with the core) and advance into the nip at a velocity different from that of the core, to achieve a surface speed differential at the point of core-cladding contact in the nip that is stated to enhance bonding; but hot as-cast strip from a continuous strip caster is not specified for the core. Further, it has been proposed, for example in U.S. Pat. No. 4,224,978, and in French Pat. No. 1,364,758, to produce a composite strip by continuously casting aluminum in a strip caster in contact with a cladding strip or strips fed through the strip caster, but these procedures involve core-cladding contact before the core metal solidifies and necessitate special or at least modified strip-casting arrangements.